Steel 4142-Prehardened Rectangular Bar 8" x 5/16" Retail Price

About 4142|Chromium-molybdenum alloy steel with 0.42% carbon. Sits right between 4140 (0.40% C) and 4145 (0.45% C). In practice, most machinists can't tell it apart from 4140 at the lathe. Same heat treatment response, same machinability, same applications. Often shows up when a distributor stocks material that falls in the overlap between 4140 and 4145 specs. Compare to 4140 (functionally identical for most purposes) and 4150 (higher carbon, higher hardness capability).

About Rectangular Bar|Solid rectangular cross-section stock. Functionally similar to flat bar but typically available in thicker, chunkier profiles. The naming varies by supplier — what one calls rectangular bar, another calls flat bar. Used for keys, gibs, wear strips, brackets, and any part where a rectangular starting profile saves setup time versus milling from plate.